The most common form of active matrix display is an active matrix liquid crystal display (AMLCD). AMLCD devices are usually made on large glass substrates that are 0.7 mm thick. Two plates are needed for a cell, so that completed displays are just over 1.4 mm thick. Mobile phone manufacturers, and some laptop computer manufacturers, require thinner and lighter displays, and completed cells can be thinned in an HF (hydrofluoric acid) solution, typically to about 0.8 mm thick. Mobile phone manufacturers ideally want the displays to be even thinner, but it has been found that cells below 0.8 mm thick made by this method are too fragile.
The HF thinning is not attractive because it is a wasteful process that uses hazardous chemicals that are difficult to dispose of safely and economically. There is also some yield loss during the etching process due to pitting of the glass.
The attractiveness of light, rugged and thin plastic AMLCDs as an alternative has long been recognized. Recently, interest in plastic displays has increased even further, partly due to the increased use of color AMLCDs in mobile phones and PDAs. There has been much research recently into AMLCDs and organic light emitting diode (OLED) displays on plastic substrates. Despite this interest, there is still a need for a plausible manufacturing route for mass production of plastic displays.
A number of different ways have been reported for the manufacture of thin film transistors (TFTs) or displays on plastic substrates.
One technique is described in WO 05/050754, in which a substrate arrangement is manufactured comprising a rigid carrier substrate and a plastic substrate over the rigid carrier substrate. The rigid carrier substrate is released from the plastic substrate after forming pixel circuits and display cells over the plastic substrate. This enables substantially conventional substrate handling, processing and cell making to be employed. WO 05/050754 also describes other known methods for fabricating electronic devices on plastic substrates.
The release process proposed in WO 05/050754 is a laser lift-off process, and the substrate material is polyimide, applied by spin-coating. One of the advantages of polyimide is its high-temperature stability.
There are, however, several drawbacks to the use of polyimide, particularly that the polyimide is yellowish, and that the spin-coated polyimide material is birefringent.
The implication of both of these drawbacks is that transmissive (transflective) displays cannot be made, nor can polarization-dependent displays be made (such as LCDs) without great difficulty.
There is therefore a need for a transparent substrate material which has high-temperature stability, is transparent, and preferably nonbirefringent. Typical transparent materials such as polycarbonate or silicones do not absorb sufficient UV energy to be lifted off by a laser release process.